A common method used to control the speed of a synchronous AC propulsion motor on a diesel-electric propulsion drive is by what means?

Correct Answer: Option B — varying the input frequency of the voltage to the motor

Controlling the speed of a synchronous AC motor requires adjusting the frequency of the stator supply. By varying the input frequency through a power converter, the rotating magnetic field speed changes, forcing the rotor to track at the new synchronous speed, which is the standard method for diesel-electric propulsion systems.